This invention pertains generally to refrigeration systems and more particularly to an improved refrigerant expansion device having means for commandably controlling an expansion valve of the system and clearing from it contaminants such as ice that accumulate in it. The invention is especially useful in cryogenic refrigeration systems employing Joule-Thomson (J-T) expansion devices.
Gases may be cooled below their liquefaction temperatures by expanding from a high pressure to a low pressure in a constant enthalpy process. When the temperature of the gas just prior to expansion is sufficiently below the inversion temperature of the gas (the temperature below which expansion results in a decrease in temperature), part of the gas undergoes a phase change upon expansion, forming a mixture of saturated liquid and saturated vapor. The expansion of gases in this manner is generally effected by so-called J-T cryostat.
A fundamental problem with J-T cryostats is clogging due to ice accumulation in the nozzle. Trace moisture in the operating gas and gas supply system is unavoidable. Depending on gas purity, J-T cryostats usually operate with the nozzle well below the front point of the gas, the temperature at which water or other contaminants freeze out of the gas. For example, at 0.1 MPa (1 atmosphere) pressure, the front point of 2 ppm water is 200K whereas the nozzle operating temperature of a cryostat operating on nitrogen is about 80K or on argon is about 90K. At higher pressures, such as inside the nozzle, the frost point is higher (e.g. 243K for 2 ppm at 20 MPa) and more water will freeze out. Clogging can occur if ice particles accumulate in the nozzle faster than they flush through. The higher the frost point or flow rate, the faster ice can accumulate. Conventional J-T cryostats typically can only operate a few minutes with 2 ppm water. This has typically limited J-T cryostats to applications requiring only short operating durations.
The present invention is directed toward the removal of this limitation so that a broader range of applications may be accommodated. For this purpose, the invention incorporates an actuator, such as a solenoid, at the warm end of the cryostat which can periodically actuate a nozzle control rod to open the nozzle, thereby mechanically dislodging any accumulated ice and causing a gas flow surge which flushes ice out of the nozzle. A return spring returns the nozzle control rod to its normal operating position when the solenoid is deactivated.
The actuator may be activated by a simple electrical power supply on command or through a timer relay to control the activation interval and hold time. In addition to clearing ice from the nozzle, the actuator may hold the nozzle open by prolonged activation or by a passive latching means to accelerate the cryostat cool down rate, or the actuator may be modulated to regulate the amount of refrigeration produced by the cryostat. Such actuated J-T cryostats may be used in a wider variety of applications than conventional J-T cryostats, particularly application requiring prolonged continuous operation and/or commandable active control of the cryostat.
Actuation is particularly well suited to General Pneumatics' patented J-T cryostat design (U.S. Pat. No. 4,631,928), which has a nozzle control rod extending the length of the cryostat.